Infobar system refactor.

This changes the ownership model of infobars so that InfoBars are long-lived and
own their InfoBarDelegates directly.  The InfoBarService pseudo-owns the
InfoBars (instead of deleting them directly, it tells them when they're unowned
and expects them to delete themselves).

This fixes leaks when infobars are closed while not visible (e.g. in a
background tab) and in general makes the system clearer and easier to reason
about.

BUG=62154
TEST=none
TBR=sky

Review URL: https://codereview.chromium.org/103993003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239110 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239110